Perforating guns that are used to perforate well casings after passing through a small diameter production tubing may be made up of individually encapsulated, pressure tight shaped explosive charges which are supported in spaced and specifically oriented positions along the length of a charge carrier section of the perforating gun. "Through tubing" perforating guns are small in diameter relative to the well casing, limiting the explosive powder load that is available for explosive preparation; consequently the penetration and phasing of the perforated holes as well as the amount and type of debris left in the well by the detonated perforating gun are important considerations.
Small diameter "through tubing" perforating guns utilizing encapsulated explosive charges are typically of two general types: in one type of perforating gun the encapsulated explosive charges are linked together as a chain leaving as debris in the well all of the metallic material in the perforating gun. Link-type perforating guns configured in a spiral firing pattern loose efficiency on the particular explosive charge shots that must fire across large diameter fluid filled casings before penetrating the formation surrounding the well casing.
The second general type of "through tubing" perforating gun utilizes in-line firing charges that are mounted on explosive resistant steel carrier strips having the capability of withstanding the explosive detonation of the charges and remaining intact so that the carrier strips may be retrieved after casing perforation activity has been accomplished. For the reason that the steel carrier strips are retrieved after firing of the perforating gun, thus only the debris of the charged cases will remain in the well after the perforating gun has been fired and retrieved. In-line firing stripguns are positioned in the well casing so that all of the charges fire with zero clearance directly into the casing, maximizing the diameter of the perforated holes in the casing and penetration of the explosive energy into the formation, yielding in general higher productivity than wells perforated with spiral phased through tubing type perforating guns. More recently, a through tubing stripgun has been developed, as indicated by U.S. Pat. No. 5,095,999 of Markel. This patent discloses an explosive resistant carrier strip which is configured to orient explosive charges so that the penetration jets of the charges are directed 45.degree. apart. Thus, when this type of stripgun establishes a line of contact with the well casing each line of explosive charges is oriented at plus or minus 22.5.degree. from the line of contact. This type of two phased through tubing perforating gun establishes a 90.degree. shot pattern which offers some advantage over the single phased in-line configuration that is normal with most stripguns.
The need for the carrier strip of a stripgun type through tubing perforating gun to remain intact when the perforating gun is detonated and the small dimension of the tubing through which the stripgun must pass to reach the downhole zone where casing perforation is desired typically limits stripguns to two phase charge orientation as shown by U.S. Pat. No. 5,095,999. If greater than two charge orientation with the type of carrier strip shown by this patent the carrier strip can be severed by the explosive detonation. It is important that the carrier strip be capable of withstanding the explosive detonation so that it can be retrieved from the well, thereby leaving only minute charge case debris to settle to the bottom of the well bore. It is desirable however to provide a through tubing stripgun capability enabling one, two or three phased charge orientation and yet insuring that the carrier strip of the stripgun will withstand the explosive detonation so that it may be retrieved from the well. It is also desirable to provide a through tubing stripgun that is provided with the capability of selectively orienting the location of the shaped charges along the length of the stripgun so that field personnel may select both the phase orientation and the vertical charge spacing as appropriate to the character of casing perforation that is desired.